Characterization of the activation antigen-specific mAb GL1 led to identification of the mouse B7-2 costimulatory molecule, which has predominant functional costimulatory role both in vivo and in vitro. Anti-B7-2 mAb inhibited accessory cell-dependent responses of T cells in vitro and in vivo, indicating that B7-2 is a functional costimulatory molecule for T cell-dependent (TD) responses. Expression of B7-2 costimulatory molecules during in vivo T-dependent B cell antibody responses correlated with somatic hypermutation and affinity maturation, and in in vivo treatment with anti B7-2 inhibited TD responses as well as memory formation and somatic mutation. The use of mice rendered deficient in both B7-1 and B7-2 has further characterized the functional requirement for B7 costimulation in antibody responses in vivo. Ongoing studies are assessing the requirement for cell autonomous function of B7 in T cell-dependent B cell activation, using mixed radiation bone marrow chimeras in which B7-deficient and wild type B cells co-exist. Transgenic mice expressing cytoplasmic deletion mutants of B7 are being generated to assess the role of functions including signal transduction through B7. T cell dependent (TD) B cell activation was analyzed in the antibody responses of mice defective in the ataxia telangiectasia (AT)mutated (Atm) gene. These mice were found to have a profound defect in vivo in Ig class switching during TD antibody responses, with most extensive defects observed in IgE and IgA response, paralleling the clinical AT syndrome. In vitro studies have identified a defect intrinsic to B lymphocytes in the ability to Ig switch in response to activation with cytokines plus LPS or anti-CD40. Molecular analysis has identified a defect in the process of Ig class switch recombination in Atm-deficient B cells, consistent with a role of the Atm product in the process of DNA break/repair involved in this recombination process. Parallel studies are in progress in Atm mice and AT patient lymphocytes to further characterize T and B cell functional status.